(TMZ)-Justin Bieber turns 19 today ... so he walked down the streets of London in half his birthday suit, lamely showing off his baby abs and his ultra-cool training boxers.

The drop-crotch pants, and the Vanilla Ice fade haircut was just too cool for school. The only saving grace ... he wasn't wearing that stupid hat.

The pic was snapped after his performance, as he walked to his hotel.

Someone needs to hang this kid upside down from a balcony.

Suge, are you listening?